I. Field of the Invention
The present invention relates to radar seeker antennas and more specifically to multiple frequency radar seeker antennas.
II. Background Art
Dual mode target seeking systems for airborne vehicles are well known in the art for operating under combinations of electro-optical, usually infrared, and radio frequency signals. Such dual mode systems involve separate systems for each frequency range incorporated to fit into a limited volume. A variety of configurations are available including parabolic reflectors, as in U.S. Pat. No. 2,972,743 of Svensson, et al. and U.S. Pat. No. 3,114,149 of Jessen, flat plate reflectors, as in U.S. Pat. No. 3,701,158 of Johnson, and image plate arrays as in U.S. Pat. No. 4,698,638 of Branigan, et al. These systems are designed to permit detection of radio frequency (RF) and infrared (IR) signals simultaneously with varying degrees of success. None of the above patents suggest, however, a means for simultaneously detecting two or more different bands of RF radiation while including an electro-optical system.
Present seeker antenna systems do not provide ready interface with both X-band fire control radar systems presently deployed and K-band systems in development. Additionally, performance requirements for small aperture dual mode (IR/RF) missiles are not met by current antenna design. Due to the small aperture size for such antennas, and the dual mode criteria, neither conventional flat plate arrays nor parabolic reflectors meet the necessary gain and sidelobe requirements. Aperture blockage due to the IR mode of operation in both types of antennas, coupled with the additional feed structure for parabolic reflectors, results in lowered gain as well as high sidelobes and, as a result, susceptibility to enemy standoff jamming techniques.
The most efficient IR/RF seeker antenna system for achieving high gain and low sidelobe requirements where volumetric constraints are prevalent is the image plate antenna. In an image plate antenna, a partially RF reflecting sheet of material is placed parallel to the reflective ground plane containing the radiating element or the element array. The image plate is constructed of a dielectric material which is one-quarter wavelength thick and is fixed by a spacer to be one-half wavelength above the ground plane. A wave entering the antenna normal to the ground plane will be reflected and then re-reflected off of the partially reflective image plate, causing the wave to travel in increments of full wavelengths so that it reaches the receiving element in phase. In dual mode (IR/RF) systems, a window which is IR transmissive and RF reflective is placed in the center of the ground plane, with the IR detector behind the RF antenna. The thickness of the image plate and its fixed spacing above the ground plane permits only a portion of one RF band to be detected by the system.
A unique small aperture antenna configuration is required which provides adequate monopulse tracking capability for both X- and K-bands in concert with an integrated (centrally located) IR sensor. One approach (General Dynamics docket no. P-1215) to attain dual band operation, given a small aperture, is an array comprised of integrated frequency selective dipoles sharing a common ground plane, in conjunction with image plate technology. This approach, utilizing the theory of images, or reflection, consists of a pure reflector surface, a dual dichroic grid image plate, two 8-element dipole arrays, and two stripline corporate feed/comparator networks. A problem encountered with this approach is to attain optimum operation, the dipoles should be positioned midway between the pure reflector and the corresponding surface of the image plate. However, this geometry creates a high standing wave field distribution in the cavity between the two reflectors with maximum field amplitude at the dipole terminals. This effect substantially increases the input impedance of the image plate-type dipole to a value approximately four times greater than that of a conventional dipole. This, in turn, causes difficulty in realizing acceptable bandwidth performance. Another issue is the dual band array's protruding radiating dipoles. The condition imposes severe aperture constriction that creates parameter degradation.
It would be desirable to have a system capable of operating at two or more different radio frequencies with high efficiency and minimum degradation while still permitting the weight- and size-economical inclusion of an electro-optical system. It is to this objective that the present invention is directed.